feat: Support Regex validation for custom attributes (#7856)

This allows a user to add/update a custom regex and a cue while defining custom attributes(Only applicable for type- text).
While adding/editing custom attributes, the values are validated against the attribute definition regex, and if it is incorrect, a cue message or default error message is shown and restricts invalid values from being saved.

Fixes: #6866

app/models/custom_attribute_definition.rb

@@ -10,6 +10,8 @@
 #  attribute_model        :integer          default("conversation_attribute")
 #  attribute_values       :jsonb
 #  default_value          :integer
+#  regex_cue              :string
+#  regex_pattern          :string
 #  created_at             :datetime         not null
 #  updated_at             :datetime         not null
 #  account_id             :bigint